1. Field
This invention relates generally to antennae and more particularly to a novel collapsible, elastic strain energy deployable helical antenna.
2. Prior Art
Helical antennas are widely used and their operating characteristics are well understood and hence need not be elaborated on.
Some helical antennas are designed to remain permaently fixed in their normal operating configuration. on the other hand, many applications require a deployable helical antenna, that is, a helical antenna which may be contracted to a collapsed configuration and extended to a deployed operating configuration. Examples of such deployable helical antennas are shown in U.S. Pat. Nos. 3,192,529; 3,524,193; 3,699,585; 3,836,979; 3,737,912; 3,129,427; and 1,106,945.
Designing a helical antenna having the capability of contraction and deployment presents certain problems whose severity increases with wavelength. These problems stem from the relationship between the overall helix diameter and cross-sectional diameter of the helical conductor or radiator and wavelength. In this regard, it is known that the optimum overall diameter of a helical antenna radiator is on the order of 0.3 times the center frequency wavelength. The optimum cross-sectional diameter of the helical conductor or radiator is on the order of 0.006 times this center frequency wavelength. At longer wavelengths, the above relationships yield helix dimensions which are too large for utilization of conventional helical antenna designs and deployment techniques.
Consider, for example, a helical antenna designed for a frequency of 120 MH.sub.z, corresponding to a wavelength of 8.5 feet. For this frequency and wavelength, the antenna helix should have an overall diameter of 2.5 feet, a length of 14 feet, and a helix cross-sectional diameter of 0.6 inches. Needless to say, a helix with these dimensions is ill-suited to use in a conventional deployable helical antenna, due in large part to the extreme stiffness of the helix resulting from its relatively large cross-sectional diameter. As a consequence of the foregoing factors, the existing deployable helical antennas are limited to relatively small helix elements and hence to relatively high frequencies.